Session Initiation Protocol (SIP) is a signaling protocol used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol to both mobile and fixed user agents (UAs). SIP is an application-layer control protocol which runs on top of several different transport protocols, such as the Transmission Control Protocol (TCP), the Internet Protocol (IP) and the User Datagram Protocol (UDP). SIP has been standardized and governed primarily by the Internet Engineering Task Force (IETF) as a protocol for setting up and managing IP Multimedia Subsystem (IMS)-based calls, and is defined by Request for Comments (RFC) 3261. SIP can be used for creating, modifying and terminating two-party (unicast) or multiparty (multicast) sessions consisting of one or several media streams. Modifications can involve changing addresses or ports, inviting more participants, and adding or deleting media streams. Other application examples include video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer and online games.
SIP invitations are used to create multimedia sessions and carry session descriptions that allow UAs to agree on a set of compatible media types. SIP uses proxy servers to help route requests to a UA's current (logical) location, authenticate and authorize UAs for services, implement provider call-routing policies, and provide features to UAs. Proxy servers, however, are merely intermediaries and are separate from the actual multimedia sessions—audio, video, and session-mode messaging—that SIP establishes. Details of the sessions are carried in the payload of SIP messages and are usually described with the Session Description Protocol (SDP) defined in RFC 4566. SIP provides a registration function that allows UAs to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.